This invention relates to a liquid flow meter of the reciprocating piston type for use in such as an oil supplying apparatus or an LPG filling apparatus.
Usually, a liquid flow meter of the reciprocating piston type comprises a housing having inlet and outlet ports, at least two pistons provided in respective cylinders in the housing, metering chambers defined on first sides of respective of the pistons, passages formed in the housing for communicating the metering chambers with the inlet and outlet ports and a valve mechanism provided in the housing and associated with the movement of the pistons for communicating either of the metering chambers with the inlet port and the other of the metering chambers with the outlet port. When the pistons are reciprocatingly moved by the pressure of liquid supplied through the inlet port, the liquid flows into and out of respective metering chambers. The number of reciprocating movements of the pistons is converted into a number of rotations of a rotating shaft by a suitable device such as a cam mechanism and is detected as, e.g., pulse signals and is transmitted to an indicator.
The flow meter of the aforementioned type has a high sensitivity and is adapted to detect a very small liquid flow, thus, it has been used widely in oil supplying apparatus or LPG filling apparatus. However, there is a shortcoming such that a change of the liquid pressure due to starting or stopping of a liquid supplying pump, pulsations of the liquid pressure caused by such as a reciprocating type liquid supplying pump, or the water hammer phenomenon due to closing of a liquid delivering valve in such as an LPG filling apparatus sometimes effect the pistons in the flow meter, whereby the pistons move momentarily and rotate the cam mechanism. As a result, the indications of the flow meter become incorrect. Heretofore, a check valve has been incorporated in a pipe line which connects the flow meter with a liquid supplying, and such check valve opens when the pressure on the upstream side of the pump exceeds a predetermined pressure, and prevents liquid flow toward the pump, so that the flow meter is not affected by such change in the liquid pressure.
In such case, it is required to mount the check valve at a mid portion of the pipe line connecting the pump with the flow meter, thereby complicating the assembling operation and necessitating an additional space for mounting the check valve.